The relationship between glycated haemoglobin levels and the risk of giant cell arteritis - a case-control study

The EULAR core dataset for observational studies in GCA does not include glycated haemoglobin (HbA_1c). A multivariable score to stratify the pre-test probability of GCA also does not include HbA_1c. There have been contradictory reports about diabetes mellitus being a risk factor for GCA. We report the first study analysing the relationship of pre-diagnosis HbA_1c with the risk of GCA.

This was a single-centre retrospective case-control study conducted in Norfolk, UK. All GCA cases were diagnosed with imaging or biopsy. Each case was assigned two age- and sex-matched controls. The primary outcome measure was the glycaemic status (HbA_1c categorized into euglycaemia, pre-diabetes or diabetes mellitus) at diagnosis between cases and controls. The HbA_1c was compared between two groups using the Mann-Whitney U test. The glycaemic categorization was compared using the χ² test.

One hundred and twelve cases and 224 controls were included. The median (interquartile range) of HbA_1c of cases and controls was 40 (37, 43) and 41 (39, 47) mmol/mol (P < 0.001), respectively. Ten of 112 cases and 52 of 224 controls had diabetes mellitus. The χ² test demonstrated a significant interaction between glycaemic state and GCA (P = 0.006). Individuals with diabetes mellitus had an odds ratio (95% CI) of 0.32 (0.13, 0.74) (P = 0.008) of having GCA compared with euglycaemic individuals.

HbA_1c in the diabetic range reduces the probability of GCA. HbA_1c should be considered in any multivariable score to calculate the risk of GCA, and in future development of diagnostic and classification criteria. There is a need for an epidemiological study looking at the possibility of a protective nature of diabetes mellitus against GCA or whether it is only a mimic.